The objective of this SBIR project is to develop spall propagation resistant turbine engine mainshaft bearings for improved reliability, safety, and performance margin of aircraft propulsion systems. New case carburized Pyrowear 675 steel has been developed for bearing races that has been shown to resist surface fatigue initiation and therefore improve the fatigue life compared to the M50 steel. However, once the initiation occurs, fatigue cracks can propagate rapidly resulting in increased spall rate in Pyrowear 675 races. This aspect of the fatigue properties of Pyrowear 675 has not been studied in great detail. In this Phase I SBIR project, UES proposes to identify the key properties of Pyrowear 675 such as microstructure, and near-surface fracture toughness that control the spall propagation rate. For this purpose, Rolling Contact Fatigue tests will be conducted under selected stress cycles, and the neighborhood of spalled area will be investigated. Spalled areas of failed bearings will also be investigated, if available. Also, computer modeling and simulation will be used to understand the role of dynamic operating conditions such as load and speed on the spall propagation.